The present invention relates to a matrix device for the detection of light radiation with individual cold screens integrated into a substrate and to its production process. The invention particularly applies to an infrared imaging system, such as a camera or an infrared sight using an infrared detector matrix having i row detectors and j column detectors, i and j being independently between 1 and 256, the matrix being illuminated by its rear face through a radiation-transparent substrate.
FIG. 1 shows a known infrared imaging system constituted in per se known manner by two matrixes, one of which 1 ensures the detection of radiation 8 and the other 3 the reading. These two matrixes 1, 3 are interconnected both mechanically and electrically by metal studs 5. The detectors 2 of detection matrix 1 are e.g. photodiodes made from a semiconductor material 7 by ion implantation or the diffusion of atoms into said material. Semiconductor material 7 is a semiconductor with a small forbidden bandwidth such as Hg.sub.1-x Cd.sub.x Te, x being between 0 and 1. By adjusting the value of x, it makes it possible to cover a wide wavelength range. This semiconductor material 7 is deposited in thin film form with a thickness of approximately 10 .mu.m by epitaxy on a substrate 6 which is transparent to the radiation 8 to be detected. For radiation 8 in the infrared range, use is made of a material such as cadmium telluride (CdTe), sapphire, gallium arsenide (GaAs) or indium antimonide (InSb).
The reading matrix 3 is made from a semiconductor material 9, such as monocrystalline silicon, by using the procedures currently employed in the production of integrated circuits. The informations relative to each detector 2 are stored, sequentially read and electric signals supplied, which are transmitted by conductors 11 to processing means. In order to reduce the incident photon flux on the detector matrix 1, it is general to associate a single screen with all the matrix 1 of detectors of a screen with each detector 2 of said matrix 1, said screens being cooled in order to limit heat radiation. These screens make it possible to reduce the incident flux in such a way that each detector 2 constituting matrix 1 has a viewing angle which is as close as possible to the numerical aperture of the optics from which the radiation 8 comes. The reduction of the photon flux makes it possible to reduce the background noise of the detectors and improve there efficiency. It also makes it possible to reduce the direct current supplied by each detector 2 and consequently increase the integration time by the reading matrix 3 for a given storable charge quantity of the elements of said reading matrix 3.
FIG. 2 shows a detector matrix 1 associated in known manner with a single cold screen. The latter is generally constituted by a ceramic 10 cooled by a liquefied gas and in the centre of which is placed the detector matrix 1, whilst around said ceramic are positioned the walls 12 of screen 4a cooled by heat contact with said ceramic 10.
The cold screen 4a with an opening 15a facing the detector matrix 1 is placed at a distance d above the same. The numerical aperture of the optics from which the radiation comes is represented by the angle 2.theta..sub.S, whilst the angle 2.theta..sub.F represents the angle under which the centre of the detector matrix 1 sees the incident radiation.
The efficiency of the cold screen is defined as the ratio between the effective solid angles .OMEGA..sub.S =nsin.sup.2 .theta..sub.S corresponding to the angle .theta..sub.S and .OMEGA..sub.F =nsin.sup.2 .theta..sub.F corresponding to the angle .theta..sub.F. Thus, the cold screen efficiency n is equal to: ##EQU1## n consequently representing the ratio between the incident light radiation reaching the aperture of the cold screen and the light radiation detected by the detection matrix.
With such a device, the larger the size of the detector matrix 1 and the smaller the aperture 15a of the cold screen 4a, the more difficult it is to obtain a good efficiency n. Moreover, the radiation detected is not uniform over the entire matrix 1 of detectors, being smaller at the edges than in the centre of the matrix.
In order to obviate these disadvantages, consideration has been given to the association of an individual cold screen with each of the detectors of a detection matrix 1. These individual cold screens appear as a system of holes made in a material and given the same spacing as the detectors.
Such individual cold screens are more particularly described in U.S. Pat. No. 3,963,926 of Jan. 9, 1975, and U.S. Pat. No. 4,446,372 of Jul. 1, 1981. Compared with the collective cold screen, individual cold screens have advantages such as the possibility of increasing the dimensions of the matrix without changing the geometry of the holes and consequently retaining the same efficiency, a homogeneous illumination over the entire detection matrix, there being no attenuation on the matrix edges and an increase in the cooling speed and consumption.
U.S. Pat. No. 3,963,926 describes a process for the production of individual cold screens associated with detectors defined in a slice of an absorbing material, such as p.sup.+ doped silicon. This silicon is etched over its entire thickness by an etching solution following the preferred locations defined by masks. The thus perforated silicon is fixed to a substrate by an epoxy resin. The upper face of the substrate supports a matrix of detectors, each hole facing one matrix detector.
This production process suffers from certain disadvantages. In particular the thus formed cold screens constitute a part fitted to the detection system. Moreover, if the material forming these screens is too thick, the walls of the openings stop part of the radiation and the efficiency of such cold screens is mediocre. Moreover, the closely spaced openings formed in the silicon weaken the structure of such cold screens.
U.S. Pat. No. 4,446,372 describes a process for the production of individual cold screens associated with detectors defined in a substrate transparent to the light radiation to be detected. These detectors are e.g. formed by ion implantation in a conduction material epitaxied on the transparent substrate. A thin opaque film of an absorbing or reflecting material with openings is deposited on the upper face of the substrate. Thus, the detectors are positioned facing the optics from which the light radiation through the substrate comes and the openings of the opaque deposit. This opaque film protects the detectors from electromagnetic radiation and improves detection. This detection has a very limited application due to the large aperture of the viewing angle of these detectors. Thus, an extreme ray of a beam reaching an opening can reach a detector close to that facing the opening.